1. Field of the Invention
This invention relates to apparatus for preventing the interception of transmitted data. More particularly, the present invention system may be employed to guarantee that data being transmitted on a directional antenna cannot be intercepted, except when the intercepting means are located directly in the main directional beam.
2. Description of the Prior Art
The problem of limiting or eliminating interception of transmitted signals is well known, and various attempts have been made to eliminate the problem.
It is well known that persons unfriendly to the United States have developed intercept techniques for intercepting, receiving and decoding transmitted sensitive data. The decoded data may be analyzed for information content, and the source and direction may be recorded for locating the transmitting source in order to direct homing devices against the source and/or to direct countermeasure devices against the transmitting source.
When the unfriendly party is able to interpose a receiving device intermediate the friendly transmitter and the remotely located friendly receiver, he is able to intercept the transmitted data. Transmitting systems have been developed which utilize transmitting antennas having main direction beam width less than one degree in cone angle width. Such systems employ line of sight frequencies and limit main beam interception to a very narrow range of altitude and direction. These systems have forced unfriendly parties to direct their intercept efforts to intercepting the data signal present in the sidelobe signals.
Prior art systems have attempted to reduce the sidelobe signals by redesigning the transmitting antenna and/or providing tapered antennas which reduce, but do not eliminate, the sidelobe data signals.
Prior art systems have employed absorbent materials around portions of the directional antenna to attenuate the magnitude of the sidelobe data signal.
The inventors of the present subject matter have analyzed numerous proposed systems for reducing the magnitude of the sidelobe signals from a directional antenna and have concluded that reduction of sidelobe gain merely forces the intercepting party to place his receiver closer to the transmitting apparatus or to develop more sensitive receiving intercept equipment in order to recover the attenuated sidelobe signals.
It would be extremely desirable to provide a transmitting system that guarantees that an unfriendly party cannot intercept transmitted data signals from sidelobe signals independent of the distance between the unfriendly receiver and the directional transmitting antenna.